1980 年 29 巻 320 号 p. 439-444
A total strain-time parametric method representing long-time stress relaxation data with different total strains at specified temperatures has been proposed. Parametric analysis was made using the observed stress relaxation data up to about 10000hr for the total strains ranging from 0.10 to 0.25 percent on 1Cr-0.5Mo-0.25V, 12Cr-1Mo-1W-0.25V and 19Cr-9Ni-1.4Mo-1.4W-Nb high temperature bolting steels. The proposed total strain-time parameter (PT) is as follows:
PT=εT-b0tb1εTa0+a1logt
where εT is the total strain, t is the time, and a0, a1, b0 and b1 are constants.
The present parameter was deduced by the following analyses: (1) the plastic strain (εP) could be expressed by means of the equation, εP=kεTm (m and k are coefficients), because a linear correlation of the data points in the coordinates of log εP and logεT was obtained; (2) both of the coefficients m and k in the above equation depended on time, and they were approximated by means of the respective equations, m=a0+a1logt and k=b0tb1.
The observed stress relaxation data for each total strain, including the data of which the relaxation curves showed an inflection of a convex nature in a certain time range, were combined into a single correlation curve with the coordinates of the residual stress (σr) and the new parameter (PT). The regression equation up to second degree, σr=c0+c1PT+c2PT2 (c0 is a constant, c1 and c2 are the regression coefficients), was applied for the master relaxation data. The estimated relaxation curves for the specified total strain calculated from the regression equation showed good agreement with the observed ones.